The invention relates to fluid flow measurement, in particular, to highly accurate flow measurement for gases.
The measurement and control of flow for liquids and gases is a major problem in various manufacturing processes. Certain industries, notably the manufacture of silicon wafers, require very accurate gas flow rates, for many different gas species.
It is well known that gas flow through a knife edge orifice follows certain laws relating flow to the gas pressures on both sides, the gas temperature, the area of the orifice and the mass density of the gas. When the pressures on both sides of the orifice are equal, there is no flow and the pressure ratio is equal to one. Flow begins when the ratio increases and generally follows the relation that flow is proportional to the product of the square root of the downstream pressure times the square root of the pressure difference. Flow in this regime is said to be sub-sonic since the gas molecules flow through the orifice at less than the speed of sound. As the pressure ratio increases further the speed of the gas molecules increases and reaches the speed of sound at a critical pressure ratio. At this point the flow is said to be sonic and it is found that flow is proportional to the upstream pressure.
Despite these well know principles it has proven difficult to reproducibly construct knife edge orifices that well consistently flow the same quantities of a given gas at the specified pressures. The reasons for this difficulty arises from several factors, including the inaccuracy in manufacturing the orifice area. Also, there is always some sidewall interaction with the gas flow such that two orifices of nominally the same area, can have flow differences as much as 30%.
An objective of the invention was to provide an improved highly accurate flow meter, based upon the gas flow versus pressure principles, for use in the especially small volumes of gas used in scientific, biomedical and engineering applications.
The present invention consists of a gas tight enclosure separated into two compartments by a crystalline membrane of silicon. There is an opening of a precise area etched into the membrane, with edges of the opening aligned along crystal planes of silicon. The membrane is sealed inside the enclosure so that gas can flow from the inlet compartment to the outlet compartment only through the orifice opening. Open passageways are provided to each compartment so that gas can flow freely into the inlet compartment from a source of gas and from the outlet compartment to a discharge destination.
Provision is made to measure the pressure inside the compartments by a transducer that supplies electrical signals proportional thereto. These signals are transmitted to control circuitry that converts them to electrical signals that correspond to the flow. The conversion factors are derived from the flow constants that define a specific crystalline orifice. In turn, the converted electrical signals are utilized to control the values shown in an alphanumeric display, thereby presenting to the viewer the desired measurement of gas flow. Corrections for temperature can be provided and a simple proportionality correction can be used to measure any gas species.